The purpose of this project is the development of a non-invasive method of measuring tissue blood flow by means of the doppler scattering of laser light. A prototype instrument has been developed which is capable of monitoring blood flow in skin and the surface of internal organs. Preliminary experiments have shown excellent results in monitoring the effects of pharmacologic agents on the blood flow of rat renal cortex, measured continuously in time, in mapping areas of infarction on the cerebral cortex of the rhesus monkey, and measuring blood flow in human skin. Work is in progress to calibrate the method against readioactive microspheres in the renal cortex, and measure flow in the renal papilla.